Cesium 133 is the element most commonly chosen for atomic clocks. To turn the cesium atomic resonance into an atomic clock, it is necessary to measure one of its transition or resonant frequencies accurately.

What element do atomic clocks use?

The Deep Space Atomic Clock uses mercury atoms; a different frequency is necessary to make those electrons change levels, and that frequency will be consistent for all mercury atoms.

Why is cesium used in atomic clocks?

And that's where caesium comes in. It has a far higher resonant frequency even than quartz - 9,192,631,770 Hz, to be precise. This is one reason Essen used the element to make the first of the next generation of clocks - the "atomic" clocks.